The present invention relates to a method of mass spectrometry and a mass spectrometer. The preferred embodiment relates to a system for and method of acquiring mass spectral data and relates to extending the dynamic range of a mass spectrometer.
There are two main types of digitiser detection systems which are used in conjunction with Time of Flight mass spectrometers namely Analogue to Digital (“ADC”) and Time to Digital (“TDC”) detector systems.
In a TDC based detector system only the arrival time of an ion is recorded. Multiple ion arrivals at substantially the same time are not recorded. In a TDC based system there is a dead-time associated with the analogue peak width of arriving ions which limits the ion flux of a species that may be counted/corrected for without leading to errors in both intensity and temporal measurement.
In an ADC based detector system the analogue signal from an ion detector is digitised and signals arising from multiple ion arrivals are recorded. However, the digitiser has a limited number of bits available. For example, an 8 bit ADC has a minimum value of 0 and a maximum value of 255 that corresponds to a given full scale deflection (“FSD”) of e.g. 1V. If a signal exceeds the maximum FSD then only a value of 255 is recorded.
It is known that at high ion arrival rates the intensity of the input analogue signal from an ion detector of an orthogonal acceleration Time of Flight mass analyser may exceed the dynamic range of a digitising ADC. This saturation may lead to errors in both the final intensity and temporal measurement of the summed data (spectrum).
U.S. Pat. No. 7,038,197 (Micromass) discloses a method of increasing the dynamic range of a Time of Flight mass spectrometer by acquiring consecutive mass spectra wherein the ion intensity is attenuated in a first mass spectrum and is unattenuated in a second mass spectrum. Peaks or regions which have exceeded the dynamic range of the detection system in the second (unattenuated) mass spectrum are then replaced with corresponding data from the first (attenuated) mass spectrum. However, this known approach suffers from the problem that the duty cycle is permanently reduced.
GB-2483322 (Maier) discloses a method of acquiring multiple groups of mass spectra with a MALDI Time of Flight mass spectrometer, wherein the energy density of the laser spot is increased in discrete steps from group to group. A mass spectrum is obtained by replacing parts of a group mass spectrum which are subject to saturation by intensity extrapolations from mass spectra of groups acquired with lower energy densities in the laser spot.
US 2002/0063205 (Micromass) discloses providing a lens which is operated in a relatively high sensitivity mode. A control system switches the lens to operate in a relatively low sensitivity mode if a predefined mass peak in a mass spectrum is determined to be saturated or approaching saturation.
EP-1901332 (Micromass) discloses an ion beam attenuator wherein the degree of attenuation can be varied by varying a mark space ratio of the ion beam attenuator. The degree of attenuation of the ion beam attenuator may be increased when it is determined that one or more mass peaks in a mass spectrum are suffering from saturation or approaching saturation.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.